In simple terms, when you fit a control valve on a process, as the valve throttles back due to low duty, the pressure after the valve naturally gets lower. In some instances, it can go sub-atmospheric.
When the steam has condensed, you obviously need to remove it. A steam trap would normally do this, but traps only work well if the condensate can fall back to the boiler feed tank by gravity (ie no back pressure)
Where gravity condensate recovery is not available, then a pumping trap can be used.
A pumping trap is a steam (or compressed air) operated pump. A set of interlocked valves allow the condensate to enter. Once the chest has filled with water, the inlet closes, the outlet opens and steam at high pressure (from your steam mains) forces out the condensate. This is pumped back to where you want it (i.e. to the boiler feed tank, or to waste or wherever you want it.
Pumping traps are mega bucks....2 to 3£K or more as opposed to a simple float trap at £200 say
Friar Tuck of Sherwood